A great number of applications in the biotechnological and pharmaceutical industry require comprehensive attention to definite removal of contaminants. Such contaminants can for example be non-eluted molecules adsorbed to the stationary phase or matrix in a chromatographic procedure, such as non-desired biomolecules or microorganisms, including for example proteins, carbohydrates, lipids, bacteria and viruses. The removal of such contaminants from the matrix is usually performed after a first elution of the desired product in order to regenerate the matrix before subsequent use. Such removal usually involves a procedure known as cleaning-in-place (CIP), wherein agents capable of eluting contaminants from the stationary phase are used. One such class of agents often used is alkaline solutions that are passed over said stationary phase. At present the most extensively used cleaning and sanitising agent is NaOH, and the concentration thereof can range from 0.1 up to e.g. 1 M, depending on the degree and nature of contamination. NaOH is known to be an effective CIP agent achieving multilog reduction of contaminants, such as microbes, proteins, lipids and nucleic acids. Another advantage of NaOH is that it can easily be disposed of without any further treatment. However, this strategy is associated with exposing the matrix for pH-values above 13. For many affinity chromatography matrices containing proteinaceous affinity ligands such alkaline environment is a very harsh condition and consequently results in decreased capacities owing to instability of the ligand to the high pH involved.
An extensive research has therefore been focussed on the development of engineered protein ligands that exhibit an improved capacity to withstand alkaline pH-values. For example, Gülich et al (Susanne Gülich, Martin Linhult, Per-Åke Nygren, Mathias Uhlén, Sophia Hober, Journal of Biotechnology 80 (2000), 169-178: Stability towards alkaline conditions can be engineered into a protein ligand) suggested protein engineering to improve the stability properties of a Streptococcal albumin-binding domain (ABD) in alkaline environments. Previously, it was shown that structural modification, such as deamidation and cleavage of the peptide backbone, of asparagine and glutamine residues in alkaline conditions is the main reason for loss of activity upon treatment in alkaline solutions, and that asparagine is the most sensitive of the two (Geiger, T., and S. Clarke. 1987. Deamidation, Isomerization, and Racemization at Asparaginyl and Aspartyl Residues in Peptides. J. Biol. Chem. 262:785-794). It is also known that the deamidation rate is highly specific and conformation dependent (Kosky, A. A., U. O. Razzaq, M. J. Treuheit, and D. N. Brems. 1999. The effects of alpha-helix on the stability of Asn residues: deamidation rates in peptides of varying helicity. Protein Sci. 8:2519-2523; Kossiakoff, A. A. 1988. Tertiary structure is a principal determinant to protein deamidation. Science. 240:191-194; and Lura, R., and V. Schirch. 1988. Role of peptide conformation in the rate and mechanism of deamidation of asparaginyl residues. Biochemistry. 27:7671-7677), and the shortest deamidation half times have been associated with the sequences—asparagine-glycine- and -asparagine-serine. Accordingly, Gülich et al created a mutant of ABD, wherein all the four aspargine residues of native ABD have been replaced by leucine (one residue), asparte (two residues) and lysine (one residue). Further, Gülich et al report that their mutant exhibits a target protein binding behaviour similar to that of the native protein, and that affinity columns containing the engineered ligand show higher binding capacities after repeated exposure to alkaline conditions than columns prepared using the parental non-engineered ligand. Thus, it is concluded therein that all four asparagine residues can be replaced without any significant effect on structure and function.
Thus, the studies performed by Gülich et al were performed on a Streptococcal albumin-binding domain. However, affinity chromatography is also used in protocols for purification of other molecules, such as immunoglobulins, e.g. for pharmaceutical applications. A particularly interesting class of affinity reagents is proteins capable of specific binding to invariable parts of an antibody molecule, such interaction being independent on the antigen-binding specificity of the antibody. Such reagents can be widely used for affinity chromatography recovery of immunoglobulins from different samples such as but not limited to serum or plasma preparations or cell culture derived feed stocks. An example of such a protein is staphylococcal protein A, containing domains capable of binding to the Fc and Fab portions of IgG immunoglobulins from different species.
Staphylococcal protein A (SpA) based reagents have due to their high affinity and selectivity found a widespread use in the field of biotechnology, e.g. in affinity chromatography for capture and purification of antibodies as well as for detection. At present, SpA-based affinity medium probably is the most widely used affinity medium for isolation of monoclonal antibodies and their fragments from different samples including industrial feed stocks from cell cultures. Accordingly, various matrices comprising protein A-ligands are commercially available, for example, in the form of native protein A (e.g. Protein A SEPHAROSE™, Amersham Biosciences, Uppsala, Sweden) and also comprised of recombinant protein A (e.g. rProtein A SEPHAROSE™, Amersham Biosciences, Uppsala, Sweden). More specifically, the genetic manipulation performed in said commercial recombinant protein A product is aimed at facilitating the attachment thereof to a support.
Accordingly, there is a need in this field to obtain protein ligands capable of binding immunoglobulins, especially via the Fc-fragments thereof, which are also tolerant to one or more cleaning procedures using alkaline agents.